Early autopilot designs employed a time based gain programmer to convert the angular glide slope error signal into a near-linear deviation signal suitable for autopilot closed loop control. This technique assumes a fixed altitude (1500 ft.) for starting the timer, a constant predetermined groundspeed and a predetermined glide slope angle. With such a system capture gains are selected for 1500 ft. altitude and are less than optimum for other altitudes. Groundspeed variations as well as glide slope angles different than those assumed also cause less than optimum gain, particularly at the end of the timer run. To improve gain control at the lower portion of the final approach the timer is reset at the middle marker.
Such system designs not only can produce a 2:1 gain variation at identical locations on the glide slope depending on glide slope angle and approach speed, but also allow for a 2:1 gain jump at the middle marker, potentially causing a control surface transient, as may be further understood from FIGS. 1, 2 and 3, showing the effect of two different airport approach conditions on a time base programmer output as a function of altitude.
Second generation autopilot designs employ radio altitude as a substitute measure for aircraft range from the glide slope transmitter. Here the assumption is that the approach terrain is relatively even and at the same elevation as the runway. Where this assumption is made, altitude above the terrain is taken as a representative measure for distance. In reality there are a number of airports with quite uneven approach terrain, particularly sharp terrain dips, and also approach terrain with a significant elevation difference relative to the runway. Such conditions produce less than optimum glide slope control gains.
Further, less than optimum glide slope control gains are produced as the result of the glide slope deviation itself, particularly at low altitudes, even for a perfectly smooth approach terrain. For example, a 20-ft. deviation under the glide slope at 100 ft. altitude produces a 20% gain deviation. A 20-ft. deviation at 100 feet may not be critical for making a successful landing, however, a more critical deviation at that altitude may render the glide slope control gains too low for recovery.
Accordingly, it is the object of this invention to provide an improved glide slope gain programming system which avoids the aforementioned problems associated with time based gain programming and radio altitude gain programming.
A further object of the invention is to provide a gain programmer which utilizes only the basic glide slope control and damping signals, readily available in a normal autopilot design, thereby avoiding the use and reliance on an additional costly information source such as a radio altimeter.
A third object of this invention is to provide a low cost high performance glide slope gain control device, making it attractive and economically feasible to design automatic glide slope control autopilots for general aviation type aircraft.